1. Field of the Invention
The present invention relates to a semiconductor integrated circuit apparatus and, more particularly, to a semiconductor integrated circuit apparatus having a programmable logic device (to be referred to as a PLD hereinafter).
2. Description of the Related Art
In recent years, semiconductor integrated circuit apparatuses capable of programming a logic gate called a PLD have been commercially available.
FIG. 1 shows a part of a PLD formed on a conventional bipolar integrated circuit. In FIG. 1, reference numerals 81 and 82 denote logic input terminals; 83 and 84, logic input inverting signal forming inverters; 85, a programming region; 86 to 88, logic circuits; and 89, a logic output terminal. The programming region 85 is arranged so that at least a specific one of fuses F respectively connected in series with a plurality of diodes D is fused to program the PLD.
This programming system is the simplest one. According to this system, on the user's side, a large current is supplied to unnecessary fuses to melt the fuses according to a desired logic circuit diagram after the shipment, or on the manufacturer's side, a laser beam is radiated onto unnecessary fuses to melt the fuses before shipment.
Another programming system is a system in which a conductive layer is formed to form a desired wiring portion.
Still another conventional programming system is available. That is, like the programming region shown in FIG. 2, a plurality of wires are arranged in vertical and lateral directions, switches SW which is passive elements are arranged at crossing points of the wires, and ON/OFF states of the switches are controlled to form desired circuit connections. In this case, reference numerals 91 and 92 denote logic input terminals; 93 and 94, logic input inverting inverters; 95, a programming region; 96 to 98, logic gate circuits; and 99, a logic output terminal. As the switch SW, memory cells such an EPROM (ultraviolet erasable programmable read-only memory) or an EEPROM (electrically erasable programmable read-only memory) which includes floating gate MOS transistors are used. Vertical and lateral wires at crossing points are connected/disconnected by writing data "0" or "1" to the memory cells.
According to the above programming system using fuses or the programming system using switches as passive elements, after a user purchases PLD products, he or she can advantageously determine logic of LSIs by himself or herself.
However, in a programming system using fuses, dust appears after the fuse is melted to form a desired logic circuit diagram. In addition, after products are shipped from a factory of a manufacturer, it cannot be estimated whether or not fuse portions to be formed by the user are good. Therefore, a non-defective ratio of 100% of the programmed products cannot be ensured. According to a desired logic circuit diagram, most of fuses must be melted in normal use, and this is troublesome. This is a reason for decreasing a non-defective ratio of programmed products.
In the above programming system using the switches SW consisting of memory cells as passive elements, since the passive elements connected to the logic input terminals 91 and 92 necessarily have stray capacitances, and the capacitances are adversely affect the PLD to delay an input signal propagation time, thereby disabling a high-speed operation of the PLD. Since a method of programming the memory cells used for the switches SW is complicated, a large-scale programming device using a personal computer or the like is required. In addition, a user must spend a long time to develop software for programming a PLD. Otherwise, the user must purchase software and a corresponding expensive programming device.
In the above programming system in which a conductive material is attached to a circuit to form a desired wiring portion, when products are shipped from a factory of a manufacturer, it cannot be estimated whether the products are good or not. Therefore, defective products are sent to a user, and a 100% non-defective ratio of the programmed products cannot be ensured. This programming system is not technically stable enough.
As described above, according to a conventional semiconductor integrated circuit apparatus, the following problems are posed. That is, when a programming system using fuses is employed, a non-defective ratio of programmed products is decreased. When a programming system using switches as passive elements is employed, the high-speed operation of a PLD is disabled.